Modeling complex plasma chemistry using the particle-in-cell code VSim

Plasma chemistry is important in low temperature plasma (LTP) discharges, particularly for processes that use complex gas chemistry, such as etching or thin-film deposition applications.  In LTP discharges, energy is primarily deposited into electrons and dissipated through collisions with the background gas, creating ions and other reactive species [1].  Most of the understanding of chemistry effects comes from fluid models, either analytical or numerical.  Kinetic models, such as particle-in-cell, can couple a collision framework to capture plasma chemistry dynamics by modeling all of the relevant interactions; however, there has been less research on using kinetic models in large-scale numerical simulations because of the computational costs.   To appropriately model discharges with complex gas mixtures, many kinetic species need to be modeled, and that can quickly make the problem computationally intractable.  Additionally, neutral gasses in the plasma evolve on the gas residence time scale, and can have a large influence on the evolution of kinetic species in a simulation.

In this paper, we report on progress on modeling complex plasma chemistry of 1D capacitive discharges using the VSim particle-in-cell code [2,3].  A Direct Simulation Monte Carlo reactions framework is used to model kinetic particle collisions between background fluids and other particles.  The reaction mechanisms for chemistries of interest (He, Ar/Cl2) were obtained from the Quantemol database [4].  Results from the Quantemol global model were used to seed the kinetic simulations to reduce the time needed to reach steady-state and to provide a point of comparison.  Differences between the kinetic simulations and results obtained from the global simulation will be discussed.

[1] M. A. Lieberman and A. J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley & Sons, Inc. (2005).

[2] C. Nieter and J. R. Cary, J. Comp. Phys. 196, 448 (2004).

[3]  www.txcorp.com/vsim

[4] J. Tennyson et al. Plasma Sources Sci. Technol. 31, 095020 (2022).